Exemplary embodiments relate to semiconductor devices. In particular, exemplary embodiments relate to an image sensor which includes a heterogeneous analog to digital converter.
A mobile device, such as a smart phone, a tablet PC, a digital camera, a MP3 player, an e-book, etc., is being used more frequently. Most mobile devices are loaded with at least one image sensor to shoot an image. Examples of the at least one image sensor includes a charge coupled device (CCD) and a complementary metal-oxide semiconductor (CMOS) image sensor.
A CCD image sensor has low noise and superior image quality in comparison to a CMOS image sensor. However, a CMOS image sensor has a simple driving type, and can be embodied by various scanning methods. Since a CMOS image sensor can integrate a signal processing circuit in a single chip, the CMOS image sensor can be easily miniaturized. Further, since a CMOS image sensor can use a CMOS process technology, the unit production cost is low. A CMOS image sensor also has very low power consumption and can be easily applied to a mobile device.
A CMOS image sensor includes an analog to digital converter for converting an image signal being sensed by an analog signal to a digital signal. A pixel array of a CMOS image sensor includes a plurality of pixels arranged in a two-dimensional matrix form. Each of the plurality of pixels outputs an image signal from light energy. Each of the plurality of pixels accumulates photocharges corresponding to the amount of light supplied through a photodiode, and outputs a pixel signal of an analog current form according to the accumulated photocharges. The pixel signal is converted into a digital signal by an analog to digital converter (ADC). Thus, a number of ADC circuits which correspond to the number of column lines of the pixel array are needed.
Various noise filtering technologies are being used in a CMOS image sensor to generate a high quality image signal. Among various kinds of noise, horizontal noise (HN) has a characteristic which is easily sensed by the human eye. Thus, various studies for reducing horizontal noise HN are under way. Horizontal noise HN can be processed in the process of converting an analog signal being output from a pixel array of a CMOS image sensor into a digital signal. However, it is difficult to effectively remove horizontal noise HN due to random noise.